Power consumption for AC motor

Discussion in 'General Electronics Chat' started by RichieG, Oct 30, 2012.

  1. RichieG

    Thread Starter New Member

    Dec 9, 2008
    Hello all ! I am trying to evaluate the electrical power consumption of a 3 phase , 1 hp, wye connected pump motor for a piece of equipment here at work. The motor literature says that it has a power factor of 71. At the electrical control box , I measure 208 vac across each phase going out to the motor, and with a clamp on amp probe, I measure about 2 amps on each of the 3 phases. To calculate electrical power consumption, do I simply take my measurements and multiply V x I for each wire and add up the three results, or is there more to it than that? Thanks!

  2. The Electrician

    AAC Fanatic!

    Oct 9, 2007
  3. panic mode

    AAC Fanatic!

    Oct 10, 2011
    certainly using wattmeter is the easiest way. if you want to calculate power then:

    where cos(θ) is phase difference between V and I. you may use value stamped on the motor. note it is 0.71, not 71.

    If you measured voltages phase to neutral (Y circuit) then

    because VLL = VLN*sqrt(3) in a 3-phase circuit
    VLL= Voltage Line to Line (phase to phase)
    VLN = Voltage Line to Neutral
  4. RichieG

    Thread Starter New Member

    Dec 9, 2008
    Thanks for all your help. Since this is just for a ballpark estimate and don't have a watt meter at my disposal, I'll just go with calculations based on my readings.
  5. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    3phase power calculations

    Total power drawn from supply W = V x I x 1.732

    actual load power given out W = V x I x 1.732 x PF(power factor)

    so in your case 208V x 2Amps x 1.732= 720.512W actual power drawn from supply.
    Motor power given out is 720.512 x 0.71 = 511.56W

    so the higher to a power factor of 1 is better as your wasting money!!!